1. Field of the Invention
This invention relates to an exposure device in which a reticle having a circuit pattern of a semiconductor device formed thereon is irradiated with a light beam emanated from a light source, such as a laser light source, for exposing the circuit pattern formed on the reticle on a semiconductor wafer.
2. Description of the Related Art
In a conventional semiconductor light exposure device, a light source of a projecting optical system o a short wavelength is employed for increasing the integration degree of a semiconductor integrated circuit. Among such light sources of the projecting optical system, there are a ultra-high pressure mercury arc lamp or an eximer laser. With the ultra-high pressure mercury arc lamp, the wavelength of the radiated light is 435.8 nm, 404.7 nm and 365 nm for the g ray, h ray and the i ray, respectively. However, the circuit pattern of fine linewidths on the order of 25 .mu.m as required of recent semiconductor integrated circuits cannot be produced even with the use of the shortest wavelength i ray. For successfully coping with the fine linewidths in the semiconductor integrated circuit, it is necessary to reduce the wavelength of the light radiated from the light source of the projecting optical system further.
In order to render it possible to form circuit patterns of finer linewidths, there has been proposed a semiconductor light exposure method known as a phase shifting method employing the ultra resolution technique. With the phase shifting method, the light transmitted through a main pattern on a reticle carrying a circuit pattern is superposed on the light transmitted through a peripheral pattern having a phase difference of 180.degree. thereto by interference for attenuating the component of the 0th order diffraction not contributing to imaging and emphasizing the 1st order diffraction necessary for pattern formation in order to improve the resolution. However, even with the semiconductor light exposure device employing the phase shifting method, it is extremely difficult to form a fine circuit pattern on the order of 0.25 .mu.m required of the highly integrated semiconductor integrated circuit.
In order to render it possible to form a fine circuit pattern of the semiconductor device, there has also been proposed a semiconductor light exposure device employing an eximer laser radiating the light of an extremely short wavelength as a light source. The eximer laser radiates a laser light by electrical discharge in a gas mixture of a rare gas and a halogen-based gas. The eximer laser radiates a laser light of an extremely short wavelength, such as 248 nm or 193 nm for the gas mixture of KrF or ArF, respectively. By employing the eximer laser as a light source of the projecting optical system, it becomes possible to achieve light exposure of a finer circuit pattern.
Since the eximer laser generates a light beam of a broader wavelength width, the semiconductor light exposure device employing the eximer laser suffers from a drawback that chromatic aberration of the projecting optical system projecting a circuit pattern formed on the reticle on a semiconductor wafer tends to be increased. For suppressing generation of chromatic aberration, the broad wavelength width of the light beam radiated from the eximer laser is decreased using a filter. By causing the light beam of the decreased wavelength width to be incident on the imaging optical system, it becomes possible to project an image of the circuit pattern on the semiconductor wafer with suppressed chromatic aberration. However, if the wavelength width of the light beam generated by the eximer laser is decreased in this manner, the ratio of the light energy employed for light exposure to the energy of the light beam generated by the eximer laser is decreased, thus lowering the energy efficiency of the semiconductor light exposure device.
On the other hand, since the eximer laser generates the light beam by pulse oscillation, it is difficult to control the volume of light radiated on the semiconductor wafer on which the circuit pattern is exposed. In addition, since the eximer laser is nonuniform in the oscillation mode of the light beam, the linewidth of the circuit pattern exposed on the semiconductor wafer is not constant.
Since the semiconductor light exposure device is designed to form an extremely fine pattern on a semiconductor wafer by light exposure, it is employed in a dust-free clean room environment. Such clean room is expensive in construction cost per unit area and in maintenance cost. The eximer laser employed for the semiconductor light exposure device is a large-sized device having an installment area on the order of 1.6 m.times.1,2 m and hence occupies a considerable space within the clean room. In addition, it is necessary for the eximer laser to exchange the gas mixture of the rare gas and the halogen-based gas employed for light beam oscillation every three days. The gas mixture is harmful to man and difficulties are met in the exchange operations.
Thus the use of the eximer laser as a light source leads to an increased size of the semiconductor light exposure device and of the clean room in which the device is installed, thus increasing the cost. The gas mixture employed needs to be exchanged, thus rendering the control of the device difficult and increasing the cost.